The invention relates to an apparatus for testing semiconductor structures, which is provided with a chuck for holding a semiconductor wafer which has a substantially unstructured underside and an upper side with structure of chips that are to be tested. The apparatus is provided with several needle holders, for externally holding contact needles which are placed on contact islands of the chips to be tested. Moreover, the chuck is provided with contacting means for the semiconductor wafer and is moved relative to the contact needles by a positioning device.
It is known that, for testing semiconductor chips, which are in a wafer assembly on a semiconductor wafer, contact needles, which serve to connect the semiconductor chips with an external test apparatus, may be placed on contact islands of the chips.
For testing chips which are separated from the wafer assembly and assembled or pre-assembled into components, it is also known that the radiation behavior of the chips may be investigated. For this, photoemissions, which generally lie in the infrared region and are generated by a circuit structure of the chips during action of electrical voltage on the chips, are determined. Subsequently, malfunctions of the chip tested are detected, for example, by a comparison with radiation images of accurately operating comparison chips or by means of generation of abnormal radiation.
To realize this test method, it is known that an infrared observation device may be used, which is directed towards an upper side of the chip. During a test, this infrared observation device is able to photograph an infrared radiation image of the chip. It is able to portray and analyze extremely weak light, which is generated particularly by malfunctioning of a semiconductor chip. Accordingly, for example, places on the chip can be recognized, which generate heat radiation as a result of a malfunction, which have defective currents due to oxide breakdowns or in gate oxide layers or which do not show the specified emissions at PN transitions, as a result of a malfunction. Such an infrared observation device can be constructed as an emission microscope or as a highly sensitive CCD camera.
However, arranging metal conductors on the chips, in some cases even in several layers, is being exploited increasingly frequently. Generally, these conductors are disposed in top layers of the chip, that is, close to the surface. However, these conductors absorb or reflect the photoemission described so strongly, that they prevent any observation of the extremely weak light radiation by an infrared observation device.
To avoid these disadvantages, it is known that the radiation behavior of the installed chip may be observed from a reverse side of the latter. In this connection, it is a disadvantage that, when a malfunction is detected, the installed chip must be discarded. With that, the expense of installing the chip, which is appreciable in some cases, is no longer of benefit.
It is therefore an object of the present invention to make it possible to detect defects of non-serviceable or not fully serviceable chips in good time and reliably before chip separation, in order to avoid an unnecessary expense of separating and installing defective chips.
Pursuant to the present invention, this objective is accomplished by a chuck having a receiving area which exposes an underside of a semiconductor wafer in an upward direction, and a receiving plane for the semiconductor wafer being disposed above a needle carrier having contact needles directed upward in such a manner as to contact the contact islands of the semiconductor chips on the downwardly directed upper side of the semiconductor wafer.
The semiconductor wafer is inserted face-down in receiving area of the chuck. Since the semiconductor wafer lies in the receiving plane and, with that, above the needle carriers, and since the needles are directed upward, it becomes possible to contact the semiconductor chips from below. The receiving area exposes the underside of the semiconductor wafer upward, so that the underside can be observed from above.
In an appropriate embodiment of the present invention, provisions are made so that the needle holder is fastened to a support which is stationary with respect to movement of a positioning device, such as an x-y mechanical stage, which moves the semiconductor wafer disposed in the receiving plane.
To realize a relative movement between the chuck and the contact needles, it is possible to move either the chuck or the contact needles. When the needle holder is stationary, a movement of the chuck is realized. Since the chuck generally is the component with the lesser weight, a more rapid positioning of the contact needles is achieved.
It is particularly advantageous if fastening means for fastening the semiconductor wafer are constructed as a mechanical and/or vacuum holding device. The semiconductor wafer is optionally clamped with a mechanical fastening device. This can be realized with simple means. On the other hand, significantly larger holding forces can be achieved with a vacuum holding device, without causing damage to the semiconductor wafer. In practice, it may be advantageous to provide a mechanical as well as a vacuum holding device. The mechanical holding device permits the semiconductor wafer to be held until the vacuum holding device acts.
In an advantageous embodiment, the mechanical fastening device includes a sliding block which, under action of a spring, engages an edge of the semiconductor wafer in such a manner that a force of the spring presses the edge of the semiconductor wafer against opposite stops. When the semiconductor wafer is inserted, the sliding block is simply pressed against the force of the spring and the semiconductor wafer inserted subsequently. When the sliding block is released, the semiconductor wafer is clamped immediately.
In a different advantageous embodiment, which is optionally used in combination with a mechanical holding device, a suction surface, on which the upper side or the underside of the semiconductor wafer is placed and which is provided with a suction groove connected to a vacuum source, is provided as a vacuum holding device. When a semiconductor wafer is placed on the suction surface, the suction groove is closed off, as a result of which the semiconductor wafer is attached by suction and held therewith when a vacuum is applied.
Furthermore, it is advisable to provide the chuck with a spacer which is connected to the x-y mechanical stage and sets a distance between the chuck and the x-y mechanical stage. The spacer makes it possible to dispose components, such as the needle carriers, between the x-y mechanical stage and the chuck. With that, the conventional construction of samplers can be maintained. An alternative includes, for example, initiating the x-y movement from outside, for example, from a remote x-y driving mechanism.
It is particularly advantageous to dispose the positioning device underneath the chuck. With that, all conventional positioning methods can continue to be used, with the sole difference that a direction of an observation device of the positioning device, which usually is constructed as a camera, changes. However, this has little effect on the positioning method.
To realize a spacing holder between the chuck and the needle holder, the chuck is constructed in plate fashion in such a manner, that it has the receiving area for a semiconductor wafer and external brackets, disposed on either side of the receiving area, at outer ends of which brackets the spacers are fastened essentially perpendicularly to the brackets.
In a preferred embodiment of the present invention, the receiving area has an opening with an opening surface, which is at least approximately congruent with a surface of the semiconductor wafer. An annular seat, which decreases an opening area, is provided at the opening on a bottom side of the chuck facing the contact needles. The seat is constructed as a suction surface and provided with an annular groove into which a vacuum duct opens. Advisably, the seat touches only an edge of the semiconductor wafer. With that, a remainder of the semiconductor wafer stays free for contacting or for observation.
It is advisable to construct the vacuum holding device in such a manner that the vacuum duct is passed through the brackets and through the spacer, that a seat, into which the spacer can be inserted and in which the vacuum connection, which can be connected with the vacuum duct of the spacer, is disposed, is provided for a spacer.
As configured above, the chuck, after being equipped with a semiconductor wafer, is placed down and, at the same time, supplied with a vacuum.
A variation of the present invention includes a fastening plate with a structure for receiving a needle holder for testing the reverse side of the semiconductor wafer and with a structure for receiving an additional receiving plate for testing the upper side of the semiconductor wafer, which can be fastened with spacers to the fastening plate and on which the needle holders can be fastened, the contact needles being directed upward for testing the reverse side and downward for testing the upper side. Accordingly, needle holder can simply be removed from the fastening plate when the testing method is changed and fastened to the receiving plate. In this case, the contact needles are changed with respect to their direction of contact, that is, the contact needles now contact the contact islands of the semiconductor chips on the upwardly directed upper side of the semiconductor wafer.